Usual small wall thicknesses of deep-drawn parts and stamped-bent parts in the sense of the invention are beneath 2000 μm. Such stainless steel parts are manufactured from very thin sheet metals by means of tensile compression reshaping or stamping-bending and sometimes take very filigree structures. Depending on the used method, parts having a varying or constant wall thickness can be produced, whereby these ones then entirely comprise a wall thickness of less than 2000 μm or they have such a wall thickness in at least some areas.
These filigree items are used in the most different fields of technique, such as for example as bearing covers in gearboxes, valve seats in ABS systems or as sample carriers for hazardous substances in high-precision measurements and are subject to extreme mechanical, thermal and chemical stress. The demand for corrosion resistant materials comprising a high hardness is therefore accordingly high.
The quality of such hardened items, in particular of such parts which have a high length diameter ratio (aspect ratio) and/or which contain nitrogen, has however been poor so far with respect to the mechanical resistance, the suitability for welding as well as the corrosion resistance. Methods based on carbon provide remedy. But they are only suitable to a limited extend for scooping deep-drawn or stamped-bent parts. Soilings are caused by the surface hardening by means of carbon, which soilings can no more economically be removed from scooping parts according to the up-to-date industrial standard. If established methods for surface hardening known from the state of the art are used for items having a very thin wall thickness and a high aspect ratio, no industrially reproducible surface layers which meet the quality requirements can be produced.
The reason for these results has over all to be seen in the partially extreme treatment conditions of the established methods.
US 2012/111454 thus shows for example a high temperature method for the carburization of rustproof steel ingots. In this method, carburization temperatures comprised between 760° C. and 1200° C. are used. Methods using such high temperatures cannot be used for the surface hardening of thin-walled deep-drawn and stamped-bent items, since they cause a thermal deformation of the partly very filigree structures and thus make them unusable.
U.S. Pat. No. 6,461,448 shows a method for the carburization of a steel article, in which the said steel article is treated in a molten alkaline bath. Such an aggressive kind of treatment causes filigree items in the sense of the invention to get partially considerable corrosion damages because of the thin wall thickness, leading to a highly inhomogeneous surface layer. Furthermore it has been proved that liquid treatments of filigree items lead to unsatisfying results because of an incomplete surface wetting.
In this connection EP 0 678 589 B1 discloses a method for the carburization of an austenitic metal. Herein, a fluorine-based gas is applied to the metal. Fluorine-based gases are highly corrosive due to their reactivity and as a result they act aggressively on the surface of the metal. While the thus caused surface removal is even desired for articles having a high wall thickness and a corresponding high amount of material, this surface removal cannot be compensated in thin-walled deep-drawn and stamped-bent items and leads to the irreversible destruction of the article. Furthermore, the gases used there are highly toxic, highly corrosive and comprise properties that are highly endangering the environment and thus they place enormous requirements on the reactor to be used, the storage and the operational safety.